1. Field of the Invention
The present invention relates to a small direct current (d.c.) motor adapted for use in office automation equipment and video equipment.
2. Description of the Prior Art
In recent years, office automation equipment and video equipment are miniaturized into a small size. This compact-size equipment requires a low-energy small d.c. motor. There are several proposals for small d.c. motors for such use. A typical example of a known small d.c. motor will be described by referring to FIGS. 1A and 1B:
The exemplary d.c. motor includes a stator section which includes a casing 22 for accommodating a magnet 21, and a rotor 23 rotatably carried on a rotary shaft 24 inside the magnet 21. The rotary shaft 24 is provided with a commutator 26 having a plurality of commutator segments 25. The commutator segments 25 are kept in contact with a brush 27 which is provided with a piece of anti-shock rubber 28. The brush 27 is supported against the casing 22 by means of a bracket 30 through a terminal plate 29.
There is a trend that as the motor is miniaturized, the contact points are proportionally reduced in size. In accordance with this trend, a metal brush becomes popular so as to reduce the resistance of the contact points to friction. In order to achieve the reliability of metal brushes, it is essential in view of reliability to reduce the friction between the commutator segments 25 and the brush 27. If the friction between the commutator segments 25 and the brush 27 is unstable, the pressure of contact therebetween varies. Any varying pressure unfavorably affects the rotating speed (rotations per minute, i.e., R.P.M.) of the motor. In addition, a gap occurs between the commutator segments 25 and the brush 27 in which electric current tends to arc, which tends to wear the brush 27 and commutator segments 25.
One proposed solution to the problem mentioned above is that the terminal plate 29 is exactly positioned with respect to the brush 27 so that the resiliency of the brush 27 can be stabilized. When the brush 27 has a relatively large resiliency, the varying contact pressure unfavorably affects the rotating speed of the motor, and the current arcing problem becomes more serious.
Another solution is that a piece of anti-shock rubber 28 is fixed to the surface of the brush 27 so as to absorb shocks arising from varying contact pressures. Thus, the brush 27 is protected against various difficulties such as shocks due to uneven contact pressure, seizure due to frictional heat, and the production of oxidized organic substances which are likely to be produced by current arcing in the air.
In accordance with the recent speed-up trend in general, a high speed rotation is demanded for electric motors. In order to facilitate the speed-up operation, it is necessary to reduce the contact pressure between the commutator segments and the brush. In addition, the miniaturization of d.c. motors is also demanded in the industry but the known d.c. motors have a brush having a relatively short resilient portion which is liable to resilient deformation. Because of the short resilient portion, the precise positioning of the terminal plate 29 or the brush 27 is difficult if it is to achieve a required range of contact pressure. Furthermore, because of the small size of the motor, the anti-shock rubber 28 cannot be fitted to the brush 27. In the end, the torque is lost, and the reliability is reduced.